One of each...
Really, there is no suitable compromise here. The 57 delivers great sound for many types of instruments, and the 58 is more oriented towards vocals.
I hear a lot of people who say the mics sound about the same. I would blame this on not so accurate monitoring. They really do have quite different sounds when applied to the same instrument or voice.
On a instrument, the 58 tends to have a sort of mushy sound. Hard to get any detail.
On voice, the 57 sounds closed in and sort of harsh.
At this point though, you need to consider your priorities for a mic. Do you need better sounding vocals or instruments.
I would favor the 57 just because it is money well spent for recording. The 58 is never going to deliver very good sound for recording vocals compared to just about any condenser mic. So, even though for right now a 58 would probably work better on your vocals, and not so good on instruments, you would wind up not using it down the road for either once you added a 57 and some kind of decent vocal mic to your collection. So it really doesn't make sense to invest in something that will not benefit you in the long run.
The 57, while not being a good choice for vocals will do okay for that, but will really shine much better on guitar cabinets, snare drums, toms, percussion, and horns (if you don't mind a little muddier sound). It is a mic that you will use for these applications even though you will may eventually have several mics to choose from.
Ed